In the audio field it has always been desirable for audio systems to be made smaller and lower cost, while at the same time producing the low frequency performance of a larger, higher cost system. This is also a goal in miniature devices, such as cell phones, tablet computers, and small multi-media loudspeakers systems. Similarly, even in larger systems, increased low frequency performance has been desired, such as in the professional, large venue loudspeaker system category, where systems are run at their full capability, singular systems that could achieve the low frequency capability in one loudspeaker enclosure that could duplicate that of two or more loudspeakers systems would be desirable. Additionally, increased low frequency capability has applications in all forms of audio reproduction that includes a low frequency range, including automotive systems, domestic audio systems, consumer audio devices, TV sound systems, home theater and surround sound systems, and music reproduction systems of all types.
A number of technologies have been suggested, each of which provide an enhancement to some aspect of lower frequencies in loudspeaker systems, but each prior art approach also comes with distinct perceptual shortcomings that in the final evaluation makes for a perceptually unconvincing alternative to a larger loudspeaker system.
Bass reproduction has two primary perceptual attributes that need to be satisfied in a small system if it is to convincingly replicate the perceived bass capability of a larger system; 1) tonal balance quality, and 2) physical impact, body-felt, quantity. And to match the capability of a larger system, both of these attributes must be achieved without audible overload distortion.
One approach of the prior art is that of systems that incorporate dynamic equalization, with the earliest of this type being U.S. Pat. No. 4,113,983, “INPUT FILTERING APPARATUS FOR LOUDSPEAKERS”, by Paul Steel, followed by later systems, such as U.S. Pat. No. 4,327,250 “DYNAMIC SPEAKER EQUALIZER”, by Daniel von Recklinghausen, U.S. Pat. No. 5,481,617, “LOUDSPEAKER ARRANGEMENT WITH FREQUENCY DEPENDENT AMPLITUDE REGULATIONS” by Egon Bjerre, U.S. Pat. No. 5,548,650 “SPEAKER EXCURSION CONTROL” by David Clarke, and U.S. Pat. No. 5,577,126, “Overload Protection Circuit for Transducers”, by Wolfgang Klippel, all of the same basic concept but with various processor control architectures.
These low frequency dynamic equalization systems basically equalize and extend low frequencies to allow a small loudspeaker to reproduce lower bass tones more accurately at small signal levels, but at larger signal levels, the majority of low frequencies are suppressed to avoid overload distortion from the low frequency woofer transducer, which also suppresses low frequency aural bass tonal level, and also, physical bass impact, at these larger signal levels, creating a thin sounding loudspeaker with substantially reduced bass impact, with very poor perceptual bass accuracy at all but very small signal levels. The bass is removed, and nothing is done to make up for, or correct the perception of poor bass reproduction at larger signal levels.
US Patent Application 2005/0207584, “SYSTEM FOR LIMITING LOUDSPEAKER DISPLACEMENT” by Andrew Bright, is another dynamic equalizer with additional signal processing, including digital implementation of the dynamic equalization. This system ultimately still suffers from similar limitations of other dynamic equalization processors reduction of physical bass impact and tonal balance quality at large signal levels without a means to correct these shortcomings.
Another approach is U.S. Patent Application 2004/0022400 “BASS COMPRESSOR” by Anthony Magrath, which uses a compressor as a ‘hard-clipper’, to limit bass and to use the distortion from the compressor such that the resultant distortion is heard as an increase in bass. This approach is problematic in that synthetic bass created by distortion has limited independent control. The requirements for optimal symmetry for compression/clipping and distortion based enhancement of low frequencies can be in conflict without isolated control, limiting the ability to independently allow the creation of an audible perceived bass response faithful to the original program source or desired bass tone increase. Also, the original physical bass impact is reduced when the compressor is activated. The application of Minnaar, “METHOD AND DEVICE FOR EXTENSION OF LOW FREQUENCY OUTPUT FROM A LOUDSPEAKER”, U.S. Patent Application 2010/0215192, is essentially a device similar in concept to Magrath but with an alternative control scheme.
Another approach to simulating greater performance from a smaller, lower powered, sound system is that of systems deploying the psychoacoustic effect of fundamental tracking or virtual pitch. An example of this type of system is typified in U.S. Pat. No. 5,668,885, “LOW FREQUENCY AUDIO CONVERSION CIRCUIT” by Mikio Oda. In these systems, a low frequency range is either inherently attenuated, or substantially removed by a static high pass filter, and an attempt is made to fill in a perception of the fundamental frequencies that were removed, by introducing harmonics of those now-missing bass frequencies, in the upper bass and lower midrange frequencies that can be more easily reproduced by the small loudspeaker and woofer transducer. The harmonics are generated statically, at all signal levels. While this can to some degree replace the bass “tones” over a narrow range of frequencies, it does not replace any of the physical bass impact of the bass, and it often applied over too wide of a frequency range to even accurately replace the tonal aspects for all the bass frequencies that are filtered out, resulting in both a total loss of physical bass impact and incomplete, or compromised, tonal quality. U.S. Pat. No. 5,930,373, “METHOD AND SYSTEM FOR ENHANCING QUALITY OF SOUND SIGNAL”, by Meir Shashoua, et al is based in the same concept of attempting to replace the tonal loss from a loudspeaker system with attenuated low frequency response, but similarly does nothing to compensate for loss of low frequency physical impact and because the artificial bass is used at small signal levels and large signal levels, it can impart an unrealistic coloration to the bass tone at all levels. Additional disclosures of this type of system are provided by Gan and Hawksford in “Perceptually-Motivated Objective Grading of Nonlinear Processing in Virtual-Bass Systems”, published in the Audio Engineering Society Journal, November 2011.
Prior art U.S. Patent Application 2007/0098182 “AUDIO FREQUENCY RANGE ADAPTATION” by R. M. Aarts, and in “High-Efficiency Low-BL Loudspeakers”, also by R. M. Aarts in the Audio Engineering Society Journal, July/August 2005, in order to reduce the size of a low frequency loudspeaker, uses a separate subwoofer box optimized to be efficient at one frequency, and uses a mapping processor to map the output of all bass frequencies below approximately 120 Hz to the one frequency. The subwoofer essentially plays just one frequency to replace all bass frequencies below 120 Hz, and this approach is applied statically at all signal levels. Because, in this system, for all bass frequencies there is only one frequency that is being reproduced, the audible output created has an unnatural bass tonal quality for all audio bass frequencies, at all signal levels. This is another system that creates a significant vacancy of frequencies throughout the bass range, with a perceptual “one note” bass effect, and it fails to recreate a realistic facsimile of a high quality low frequency system, and is relegated to use in low fidelity systems.
A common enclosure design is a sealed, acoustic suspension enclosure which exhibits increased diaphragm displacement throughout the lower frequencies without any resonant chamber based frequency and range of reduced diaphragm displacement, and tend to be a poor performer for low frequency acoustic output relative to diaphragm displacement. Historically vented box loudspeakers have been known to provide greater output at a vented box tuning frequency, for a given diaphragm displacement, but exhibit significantly greater diaphragm displacement for all frequencies below the tuning frequency and also for a band of frequencies above the tuning frequency, and therefore are limited in the ability to take advantage of the reduced diaphragm displacement at the vented box tuning to produce greater output over the full range of bass frequencies. These systems are disclosed in “Vented-Box Loudspeaker Systems Part 1 and Part 2”, in the Journal of the Audio Engineering Society, June and July/August 1973 issues, by Richard H. Small.
Single and multi-tuned bandpass enclosures attempt to improve output capability but still suffer from increased diaphragm displacement above the lowest tuning frequency, thereby limiting total output capability to the weakest, high displacement frequencies which substantially override the gains from the reduced displacement frequencies. These systems are disclosed in “An Introduction to Band-Pass Loudspeaker Systems” by Earl R. Geddes, in the Journal of the Audio Engineering Society, May 1989 issue.
Additional loudspeaker enclosure designs have been introduced to attempt to create smaller low frequency systems that can reproduce lower frequencies and play them louder without distortion, such as U.S. Pat. No. 4,628,528, “PRESSURE WAVE TRANSDUCING” by Amar Bose and U.S. Pat. No. 5,092,424 “ELECTROACOUSTICAL TRANSDUCING WITH AT LEAST THREE CASCADED SUBCHAMBERS”, by William Schreiber, et al. These can provide reduced diaphragm motion at a few narrow frequency ranges, but have other portions of the bass range where the diaphragm motion is significantly greater, and may have even poorer performance than an acoustic suspension system at those high displacement frequencies and cannot support full output bass without overloading except at a few narrow frequencies, and so the maximum level capability of the systems over the full range of low frequencies is substantially limited to the output capability of the highest displacement frequency ranges and the reduced displacements in the narrow ranges cannot fully contribute to maximizing undistorted bass output through the bass frequency range.
Other systems have developed variations on these themes, primarily introducing more efficient processing algorithms and refinements, but none that change the fundamental limitations of these approaches to provide large bass system performance in a small device without significantly compromising perceived sound quality and quantity.
There is a still an unfulfilled need for an approach that can improve the perceived bass quantity and extension for a given size of low frequency system, without significantly compromising the bass quality and fidelity.
It would be desirable to have a low frequency loudspeaker enclosure and signal processing system that can maximize the low frequency capability and increase the sound quantity in the low frequency range for a given size enclosure while substantially maintaining the perception of sound quality, providing perceived tonal accuracy and realistic physical bass impact.
Applicant hereby incorporates herein by reference any U.S. patents and U.S. patent applications, and technical papers cited or referred to in this application to the extent the prior disclosure is consistent herewith, and to the extent inconsistent, this later disclosure shall control.